1. Field of the Invention
The invention relates to semiconductor fabrication, and more particularly to improving manufacturing yield by modifying design data to create three-dimensional structures of robust design.
2. Background Description
The manufacturing yield of a manufacturing process for semiconductor devices may be affected by a variety of factors. For example, the semiconductor fabrication process may produce a defective circuit due to contamination during fabrication. Such contamination may include foreign particles finding their way onto a surface of a circuit under fabrication. Such a foreign particle can interfere with the manufacturing process so that subsequent steps are not properly completed leading to a malfunctioning device. Malfunctions may include, for example, the shorting of wires to one another or a broken wire on a particular layer of the circuit.
Other factors which may negatively impact semiconductor manufacturing yield may include certain aspects of the device's design. For example, where a device's design specifies wires which are narrower than the narrowest wire that the fabrication process can reliably fabricate, the wire may be formed with a gap therein leading to a permanently open circuit. Another example of a design defect includes wires which are too close together. Where a wire is too close to an adjacent wire, there may be bleeding of one wire to the adjacent wire causing electrical contact between the two. Such unwanted electrical contact causes a permanent short in the circuit and prevents the circuit from functioning properly. Ground-rule checking of a semiconductor design should in practice eliminate errors of this kind, but there are situations in which a design rule is waived, meaning that such a violation maybe permitted.
By contrast, certain geometrical configurations of layout elements can obey all design rules and yet still be difficult to manufacture reliably. For example, types of designs which are prone to producing lowered manufacturing yield include designs which align one particular structure on a first layer with a second particular kind of structure on a second layer. Such problematic designs may lead to one structure affecting the formation of the second structure in a negative way.
In other words, in the manufacture of VLSI integrated circuits, certain three-dimensional structures can be difficult to build reliably. For example, structures such as wires at minimum spacing on a metal layer Mx over wide or large wires on the metal layer Mx-1. Note that this minimum spacing value on layer Mx is typically dependent on the width of the Mx wires, so that minimum-width wires require a smaller wire-to-wire spacing value than is required between wires of larger width. Another example of a difficult to manufacture structure includes wires at minimum spacing on Mx over a trench between two wide wires on Mx-1. In particular, during planarization, the polishing of wide wires on Mx-1 causes a local dishing of the surface (so-called “induced topography”). This topographic variation can cause shorting of the minimum-spaced wires in a subsequent metal layer.
It should be noted that traditional design rules do not prohibit the aforementioned problematic structures. Furthermore, routing programs typically do not recognize or try to avoid such interaction of structures from one layer to the next. Also, any cheesing steps to make a more uniform distribution between metal and dielectric on a layer does not necessarily alleviate the topographic problems because the wider wires can fall under the threshold shape width for which cheesing will be applied. It should also be noted that random-defect analysis does not reveal the presence of the problematic metal structures. Rather, such three-dimensional structures cause systematic yield loss, where, regardless of their physical location in the design of the circuit, they are consistently difficult to manufacture. Furthermore, there have been no automated techniques for treating the systematic defects associated with the induced-topography defects.
Accordingly, manufacturing problems can result from certain two-dimensional structures which should be avoided, such as one wire too close to a second wire, as well as certain three-dimensional structures encompassing multiple layers of a device which should also be avoided. For example, where minimum-spaced wires of an upper layer cross over a large wire of a lower layer, the flatness or planarity of the lower layer may be critical for proper small wire formation on the upper layer. Also, it is well known that where there is a relatively large wire formed by, for example, a damascene process in an oxide or other type of insulator, it may be difficult to form a planar surface across the dielectric and wire surface.
For example, in a damascene process, the lack of planarity across the wire and dielectric is caused primarily during the last step of the process which typically includes a chemical/mechanical polishing (CMP) step. Thus, during the CMP process, because there is such a relatively large expanse of metal for the wide or large wire surface compared to the surface area of the surrounding dielectric, the metal may become dished during the polishing process, leading to a non-planar surface.
Additionally, at the boundary between a wide metal line and adjacent dielectric material, the metal may become slightly recessed below the level of the surrounding dielectric. Thus, the dielectric will then protrude slightly above the surface of the metal large wire and lead to the formation of an unwanted trench at the edges of the metal line. The non-planarity of the dished metal line, and/or the trench, will cause, for example, non-planarity in subsequent layers formed above this lower layer. The non-planarity may then lead to metal wires formed on the subsequent layers being improperly formed and shorting to one another.
Traditional methods of mitigating the effects of such non-planar surfaces on a first layer interfering with the proper formation of structures on a subsequent upper layer traditionally focus on fabrication process changes on a single layer with no consideration given to altering the design of a first layer to solve fabrication problems of a second layer. Thus, a two-dimensional approach is traditionally taken. For example, where an unwanted trench is typically formed at the edge of a wide metal wire, the size of the trench is reduced, or the trench is eliminated altogether, by carefully adjusting the parameters of the polishing step of the CMP process. Accordingly, parameters of the polishing step may be adjusted by either altering the composition of the chemicals used during the CMP process or by altering the length of time of the polishing, etc. However, there must be a balance which has to be achieved between over-polishing—which can lead to trench formations and subsequent problems on higher layers—and under-polishing, which additionally causes non-planarity on the lower layer.
Referring to FIG. 1, for example, small metal wires 12 fabricated at, or close to, the minimum spacing possible on an upper layer are shown crossing a wide wire 14 on a lower layer. Thus, minimum-spaced wires 12 are wires which are manufactured near the lower limits of resolution of the manufacturing process. It should be noted that the wide wire 14 could potentially be dished during the CMP step of the damascene-forming process. Accordingly, where the minimum-spaced wires 12 lie above the wide wire 14, the non-planar surface below may inhibit the proper formation of the wires 12.
Referring still to FIG. 1, a dielectric 15 surrounds the wide wire 14. The dielectric 15 is typically an oxide or a nitride type dielectric. The wide wire 14 is typically copper, but may also be constructed from aluminum, as well as other conductors. Likewise, the minimum-spaced wires 12 are typically copper but may also be constructed from aluminum as well as other conductors.
Accordingly, the minimum-spaced wires 12 pass across the top of the wide wire 14 and then pass onto the dielectric 15. Because the damascene process used to form the wide wire 14 typically causes some dishing of the surface of the wide wire 14, the wide wire 14 and surrounding dielectric 15 may not be planar. Consequently, when an adjacent layer is formed on top of the wide wire 14 which includes, for example, minimum-spaced wires 12 imaged in a photolithographic process, the non-planar surface of the lower layer interferes with the material removal through planarization of the subsequent metal layer, resulting in a shorting of the metal material or of the liner material that is deposited between the metal and the dielectric.